After the discovery of high-temperature oxide (ceramic) superconductors which become superconducting above liquid nitrogen temperature (77 K), fundamental and application researches have been developed actively.
One of the most important problems to be solved in the application research of high temperature oxide superconductors is to make sure the manufacturing technique of a superconducting film of desired grain boundary characteristics of a predetermined pattern precisely.
Many techniques such as sputtering, vacuum deposition (reactive deposition, MBE, ICB, laser deposition or the like), MO-CVD, spray pyrolysis, screen printing and sol gel have been used to prepare a superconductor film. These techniques have their respective characteristics. For example, sputtering, vacuum deposition and MO-CVD can be used to manufacture a monocrystalline or quasi-monocrystalline thin film. Thus, they can produce a thin film of high quality so that they can be used to manufacture an electronic device including a high temperature superconductor instead of a metallic superconductor which has been used before. However, the area of a film manufactured by these techniques is small. Further, a vacuum pump is needed and an apparatus for these techniques becomes on a large scale. On the other hand, techniques such as spray pyrolysis, screen printing and sol gel are not suitable to produce a monocrystalline thin film, but they can be used widely for applications with use of polycrystalline material such as a non-resistance printing circuit board, a magnetic shield, a wire and a sensor. However, these techniques are based on complicated chemical processes for forming a film, so that they are not good as to the reproducibility or reliability. Thus, they cannot be used to produce a device such as a magnetic sensor, a photosensor or a logic device which makes use of the weak-coupling at crystal grain boundaries. Further, with these techniques it is hard to produce a film of a complicated pattern of wide area on a board.
Especially, high temperature superconductors have a short coherence length and a small carrier concentration. Therefore, they have a characteristic that crystal grains are easy to be coupled weakly to each other at grain boundaries, like a Josephson junction. By using this effect, a wide range of applications will be expected to be developed. For example, a device such as a magnetic sensor (see, for example, H. Najima et al., Jpn. J. Appl. Phys. 27 (1988) 746, a photosensor, or a logic device can be realized. Unfortunately, a film of high temperature superconductor has generally grain boundaries of porous structure, so that it is difficult to make a film of a predetermined pattern by using a general photoetching process. That is, in the photoetching processing, a resist or an etchant penetrates the film through grain boundaries, so that the patterning of the film may not be performed precisely. Thus, a device which makes use of grain boundary characteristics of high temperature superconductor is difficult to be manufactured so as to have a minute pattern.
Recently, a fundamental technique which makes use of electrophoresis to form a superconducting film was reported (for example, N. Koura, Denki Kagaku 56, 208 (1988), and C. T. Chu and B. Dunn, Appl. Phys. Lett. 55, 492 (1989)).
In this electrophoretic deposition technique, fine powders of superconducting material, which have been dispersed in an organic solvent, are deposited on a substrate kept at a negative electric voltage against a counter electrode, and the deposited film is sintered with the use of heat treatment to form a superconducting film.
A superconducting film manufactured by using the electrophoretic deposition technique has many superior potentials on the application of high temperature superconductors. However, there are many unknown points in the mechanism, and the guidelines to control the characteristics of a film are insufficient. That is, there remain many problems to be solved.
Further, an electrode such as a precious silver plate was used as an electrically conductive substrate on which fine powders of oxide superconductor are deposited. However, silver is expensive. Therefore, a less expensive material is desirable.
Still further, as mentioned above, a superconducting film manufactured by using electrophoretic deposition techniques was also difficult to be processed to have a desired pattern by using the photoetching technique because it has porous grain boundaries. Therefore, it was impossible to design a device having a minute pattern so as to make use of grain boundary characteristics of high temperature superconductors. It is desirable that a film is composed of crystal grains of similar size and similar grain boundary characteristics to each other.